Thanks everyone for your messages and generous offer of help. I have just tried to use the freebmd website, but not sure if I am doing it right, will keep perservering though.
My nan's maiden name was Edna May Strudwick, born on 18 November 1928 at East End Mothers' Lying In Home, Commercial Road, Stepney, London. Edna's mother's maiden name was Ethel Maud King, she married Albert Edward Strudwick. Ethel & Albert married on 2 June 1928 in Bethnel Green. Oh yes, we have noted that there isn't 9 months between marriage and Edna being born!
Before Ethel got married she lived at 14 Belhaven Street, Bethnel Green.
Ethel was widowed in 1936 when Albert died.
Ethel remarried on 23 December 1939 in Poplar, marrying a James Thomas. At the time of this marriage, Ethel was living at 41 Ettrick Street, Poplar.
Before my nan passed, she mentioned this other sister. She was too ill to question, but we are not sure if it the father was Albert Strudwick or James Thomas. So not sure how to check how Ethel King, Ethel Strudwick or Ethel Thomas had children. Sorry for the novel, I hope I have provided enough details.